I'm currently seeing two different guys, both of whom I met online. The first is fun and seems interested in me, but I can't tell how serious he is about settling down. The second is a little older and more established, but because of his job, we won't be able to see much of each other for the next few months. How much time or attention do I give to either or both of them, and how do I tell if they're ready to make a commitment?
Who to Pursue
As long as you're having fun with the first guy, keep having fun. As long as that doesn't get too serious, when the second guy is back in town, you can explore your options there, too. Until either one of them brings up anything about exclusivity, you don't have to bring it up, so there's no expectation that there are no other guys in the picture. That also means you should assume they are seeing other women as well. As long as you're okay with where things stand, enjoy whatever time you have with either of them.
On the other hand, you mention settling down, so it sounds like that's important to you. An unknown time frame with two different guys doesn't necessarily get you any closer to a long-term relationship, and giving attention to both guys probably doesn't leave much time for meeting other potential mates online. You could bide your time until the second guy is available again and then reassess the comparisons between the two, but be prepared to realize in a couple of months that neither of them is going to be the father of your children.
Your best option is to keep your options open. Don't tie yourself down to the fun guy and don't put too much stock in the unavailable guy. Keep browsing online dating sites and see if there's a third (or fourth!) guy to add to the mix. While dating multiple people at once can be tough on your schedule, your memory and, depending on your style, your wallet, it sounds like you're already benefiting from being able to compare different personalities — so there's no reason not to see who else is out there.
Ideally, we'll each meet the perfect soul mate who fulfills every characteristic on our long, unrealistic checklists of what we're looking for in a partner and live happily every after. It's never that simple, though. So while I do suggest that you don't necessarily settle for the two guys currently in the picture, I also caution against assembling a smorgasbord of men who each fill one or two criteria but none comprise the right whole package. Be open to meeting someone who doesn't match every item on your checklist. As long as you don't care what anyone else thinks, you'll know he's the one to pursue.